This invention relates to semiconductor packaging and, particularly, to attachment of the semiconductor die onto the lead frame or substrate in packaging power devices.
In power packages, the die is attached to the leadframe by affixing the backside of the die to the die paddle of the lead frame. Typically, electrical current is conducted from the backside of the die to the die paddle. Power devices consume a considerable amount of energy, and dissipate great amounts of heat. Accordingly, preferred die attach materials are highly heat conductive and electrically conductive and, generally, solders are preferred has die attach materials for power packages.
A conventional die attach method using a soft solder includes three in-line processes, namely: dispensing the solder, “spanking” the solder, and mounting the die onto the spanked solder. In the solder dispensing process, the solder is drawn into a wire, and a solder feeder using a vertical feed movement dispenses a droplet of molten solder from a supply onto a nickel-plated or bare copper lead frame die paddle in a heat tunnel of a die attach machine. In the solder spanking process, a “spanker” is used to flatten the molten droplet of solder so as to achieve an even spread of liquid solder in a desired rectangular shape over the die attach surface of the die paddle. The “spanked” solder typically has a size about 20% greater than the die size. In the die mounting process, a suction operated pick-and-place tool is used to pick up a singulated die from a sawn wafer and position the die backside downward upon the spanked solder. The solder is permitted to solidify in a cooling zone, completing the attachment.
A critical part of such a conventional process is achieving an even spread of solder, using a minimum quantity of solder. Die attach performance is degraded by solder voids, inadequate solder coverage, and nonuniform bond line thickness, and typically the process requires a lengthy setup time for spanker process buyoff to avoid these defects. Longer conversion in-line process times result in loss of productivity.
In one approach to improving the conventional process, a soft solder dispensing module is used to dispense the molten solder in a distributed pattern over the die attach surface of the die paddle. This requires additional setup and maintenance of the dispensing module, and the cost of maintenance can be high. Frequent change of the pattern mold casing may be required, and a cleaning station must be provided to clean solder residues in the pattern mold casing.
In another approach to improving the conventional process, a solder paste is employed in place of molten solder. The solder paste is dispensed onto the die attach surface of the die paddle, using a syringe dispenser or a “shower head” dispenser, or printing through a stencil. Then a pick-and-place tool is used to press the die onto the dispensed solder paste, and the solder paste is reflowed to complete the attachment. The dispenser can become clogged with solder paste, requiring cleaning. Solder paste can be difficult to control in processing; solder paste “bleed” can result in solder contamination on nearby contact sites, and the paste can creep onto and contaminate the top (active) surface of the die. Uniform bond line thickness can be difficult to obtain. Additional workstations are required, notably flux and in-line reflow stations, making the process more complex.
Various particular approaches to die attach have been proposed.
For example, U.S. Pat. No. 5,177,032 describes attaching a semiconductor die to a lead frame using a thermoplastic covered carrier tape. The thermoplastic is softened to a desired state by heating, the and the die is bonded to the lead frame by the thermoplastic. This approach is simpler than conventional methods using epoxy or a eutectic solder paste, but it is not useful for power packages. The thermoplastic material is not electrically conductive, so it cannot be used for die attach where electrical conduction from the backside of the die to the die paddle is required.
U.S. Pat. No. 5,904,504 describes attaching a semiconductor device to a printed layer of adhesive on the lead frame paddle using a recess template to create a dimensionally controlled imprint with a predetermined quantity of die attach adhesive. This approach includes melting the adhesive in a reservoir, requiring further equipment modification and setup control, and necessitating periodic cleaning of the reservoir. These requirements entail additional cost.
U.S. Pat. No. 4,454,840 describes a die attach adhesive that includes of a plurality of spacers in a suspension of silver filled glass. A solvent is driven from the silver filled glass during the die attach operation, causing the volume of the silver filled glass to decrease. The spacers are partially melted during thermal heating, so that the thickness of the spacers decreases, eventually forming a bond. This approach is costly, and requires extra handling of the solder material and other apparatus for die attach machine setup.
U.S. Pat. No. 6,525,423 describes using a plurality of wires as rigid spacers to support the die attach material, as an inexpensive means for achieving a uniform and consistent spacing between the die and the substrate. This approach is not suitable for power packages, as the wire spacers will restrict the flow of molten solder.
U.S. Pat. No. 6,524,891 describes pressure curing the die attach material, said to reduce voids in the die attach bondline. The additional step of pressure curing is a non-value added process for volume production manufacturing. An additional pressure chamber is also required to support this invented method.
Conventional soft solders include lead as a component. There is significant public concern regarding lead (Pb) and the use of lead-based materials, owing to effects on human health and the environment. Efforts are underway to developing suitable lead-free replacements for soft solder alloys. Most of the suitable lead-free alloys have generally higher reflow temperatures (for example, about 260° C.) than conventional lead-based soft solders (for example, about 220° C.). This can present challenges for manufacture of packages containing temperature sensitive semiconductor devices.
Certain power devices are thermally sensitive, and the die attach process for such devices must be conducted in a temperature regime that does not damage the die. For example, die attach for certain thermal sensitive metal-oxide-semiconductor field effect transistor (MOSFET) devices must be conducted at a mounting temperature below 300° C. As the die size of the MOSFET device is generally smaller than that of a bipolar junction transistor or other field effect transistor (less die area is required owing to high density), the integrated circuit built in the die is more sensitive to thermal induced stresses. Common thermally related defects are forward second breakdown, ionic contamination and hot surface charge spreading on the device surface.
Forward second breakdown occurs due to thermal runaway at a point in the aggregate transistors that operate under different conditions in a MOSFET. Because the resistivity of the silicon increases with temperature from ambient (in the die attach heat tunnel), the temperature rise causes the collector to become sticky, or may induce a thermal runaway defect in the device even before packaging assembly is completed.
Often the MOSFET (n-channel) is covered a with phosphosilicate glass (PSG) film so as to stabilize the threshold voltage against chances of ionic contaminants (sodium) due to high heat. Mobile charged ions present within the oxide or at the device-oxide interface will influence the device threshold voltage. Extra positive charge at the SiO2 interface induces extra negative voltage in the n-channel, resulting in a decrease in the threshold voltage of the device. Charged devices will remain charged over a period of time, and the failure can start from the heat applied during the package assembly.
Surface charge spreading in MOSFET devices as a result of high ambient heat involves a lateral spreading of ionic charge from the biased metal conductors along the oxide layer or through moisture on the device surface. An inversion layer outside the active region of the transistor develops due to the charge forming a conduction path between the two diffused regions by an extension of the p-n junction through a high leakage region, resulting in leakage currents between the neighboring conductors. The rate of charge spreading increases with the temperature increment in the ambient, and can be stored in the inversion layer for some time. Overheating during die mounting can destroy the mounted MOSFET unit.
Accordingly, it is important in packaging temperature sensitive devices such as MOSFET power devices, to carry out the die attach at lower temperatures (below 300° C.) for some devices, and at the same time to achieve a uniform and desirable bond line thickness condition with higher die mounting units per hour.